Cellulosic fibers are a basic component of absorbent products such as diapers. The ability of an absorbent product containing cellulosic fibers to initially acquire and distribute liquid will generally depend on the product's dry bulk and capillary structure. However, the ability of a product to acquire additional liquid on subsequent insults will depend on the product's wet bulk. Cellulosic fibers, although absorbent, tend to collapse on wetting and to retain absorbed liquid near the point of liquid insult. The inability of wetted cellulosic fibers in absorbent products to further acquire and distribute liquid to sites remote from liquid insult can be attributed to a diminished acquisition rate due in part to the loss of fiber bulk associated with liquid absorption. Absorbent products made from cellulosic fluff pulp, a form of cellulosic fibers having an extremely high void volume, lose bulk on liquid acquisition and the ability to further wick and acquire liquid, causing local saturation.
Crosslinked cellulosic fibers generally have enhanced wet bulk compared to uncrosslinked fibers. The enhanced bulk is a consequence of the stiffness, twist, and curl imparted to the fiber as a result of crosslinking. Accordingly, crosslinked fibers are advantageously incorporated into absorbent products to enhance their wet bulk and liquid acquisition rate and to also reduce rewet.
Some of the first crosslinked cellulosic fibers were prepared by treating cellulosic fibers with formaldehyde and various formaldehyde addition products. See, for example, U.S. Pat. No. 3,224,926; U.S. Pat. No. 3,241,553; U.S. Pat. No. 3,932,209; U.S. Pat. No. 4,035,147; and U.S. Pat. No. 3,756,913. Unfortunately, the irritating effect of formaldehyde vapor on the eyes and skin is a marked disadvantage of the fibers. In addition, such crosslinked fibers typically exhibit objectionable odor and have low fiber brightness.
Alternatives to formaldehyde and formaldehyde addition product crosslinking agents have been developed. Among these are dialdehyde crosslinking agents. See, for example, U.S. Pat. No. 4,822,453, which describes absorbent structures containing individualized, crosslinked fibers, wherein the crosslinking agent is selected from the group consisting of C2–C9 dialdehydes, with glutaraldehyde being preferred. The reference appears to overcome many of the disadvantages associated with formaldehyde and/or formaldehyde addition products. However, the cost associated with producing fibers crosslinked with dialdehyde crosslinking agents such as glutaraldehyde is considered too high to result in significant commercial success. Therefore, further efforts have been made to improve fiber properties such as color and odor.
Polycarboxylic acids have been used to crosslink cellulosic fibers. See, for example, U.S. Pat. No. 5,137,537; U.S. Pat. No. 5,183,707; and U.S. Pat. No. 5,190,563. These references describe absorbent structures containing individualized cellulosic fibers crosslinked with a C2–C9 polycarboxylic acid. The ester crosslink bonds formed by the polycarboxylic acid crosslinking agents differ from the acetal crosslink bonds that result from the mono- and di-aldehyde crosslinking agents. Absorbent structures made from these individualized, ester-crosslinked fibers exhibit increased dry and wet resilience and have improved responsiveness to wetting relative to structures containing uncrosslinked fibers. Furthermore, the preferred polycarboxylic crosslinking agent, citric acid, is available in large quantities at relatively low prices making it commercially competitive with formaldehyde and formaldehyde addition products. Unfortunately, the preferred C2–C9 crosslinking agent, citric acid, can cause discoloration (i.e., yellowing) of the white cellulosic fibers when the treated fibers are cured at the elevated temperatures required for crosslinking It is known that decomposition of citric acid yields aconitic acid, itaconic acid, citraconic acid, and mesaconic acid. Yellowing may be due to the chromophores produced as a result of the conjugated double bonds produced or due to reactions with the double bonds. In addition, unpleasant odors can also be associated with the use of α-hydroxy polycarboxylic acids such as citric acid. The above-noted references do not describe processes that reduce the odor or increase the brightness of the treated fibers.
We have found that the color and brightness properties of citric acid and other α-hydroxypolycarboxylic acids crosslinked cellulosic fibers could be improved by crosslinking cellulosic fibers with the crosslinking agent in the presence of a polyol. It has now been discovered that further increases in color and brightness can be obtained by contacting these crosslinked fibers with an oxidizing bleaching agent (e.g, hydrogen peroxide). Alternatively, the fibers can be contacted with an aqueous solution containing hydrogen peroxide or an aqueous solution containing sodium hydroxide and hydrogen peroxide.
Although some disadvantages related to brightness and color associated with crosslinked cellulosic fibers have been addressed, a need remains for cellulosic fibers having the advantages of bulk, liquid acquisition, and rewet associated with crosslinked cellulosic fibers without the disadvantages related to diminished fiber brightness and color. The present application seeks to fulfill these needs and provides further related advantages.